This invention relates generally to affecting reactions between blast furnace gas and minerals present in the blast furnace shaft, and relates to the distribution of minerals with relation to the formation of molten slag. There are also factors related to dust suppression in iron ore agglomerate handling and transport.
Iron oxide pellets are normally used alone or together with natural lump ores or sinter as iron units in blast furnaces. In the high temperature region of the furnace, above approximately 1000° C., reduction of iron oxide to metallic iron accelerates rapidly. It has been found during this rapid reduction step that iron ore agglomerates may cluster due to iron-iron sintering or the formation of low melting point surface slag. As the temperatures increase further, slag forming material in the agglomerates begin to melt and eventually exude from the agglomerates. The primary slags tend to be acidic in nature. These so-called primary slags contain residual FeO which is then reduced via contact with reducing gas or carbon. Iron in contact with carbon carburises and melts. Slags formed in the primary process react with other lumpy slag formers in the burden to form secondary slags, and then eventually with residual coke ash to form the final slag that is tapped from the furnace. It has been found that this melting process—including slag and iron meltdown and carburisation—affects greatly the stability in the melting zone and hearth of the furnace, and can affect gas flow. Maintaining fluid slags throughout the process is critical to stable operation. This is especially important for furnaces operating with very low slag volumes as the basicity of the secondary slag in the ore layer becomes higher with greater risk of extreme differences in melting temperatures between primary slag and secondary slag. In some instances, due to the endothermic reduction of FeO and melting of iron, slags may refreeze blocking gas flow through the ore layer and delaying further reduction and melting. Improving the distribution of slag formers reduces the extremes in differences in slag melting temperatures.
In the very high temperatures at the tuyeres and hearth, some of the alkalis (potassium and sodium) entering with the charge material are reduced and vaporized, rising with the gas in the shaft. As the alkalis rise, they react first with acid components in the burden which are well known to capture alkali. Alkalis not captured in the acid components continue to ascend and are deposited as carbonates and cyanides. These depositions are known to cause scaffolding, hanging and also react with the refractory lining of the furnace. Also, the presence of alkali in reducing gas has been shown to cause degradation of coke and iron ore agglomerates which results in permeability problems in the packed bed. The degree of alkali circulation and the behaviour of the coke and ferrous burden in the presence of alkali are constant sources of concern in blast furnace operations.
The phenomena of clustering of ores, poor slag formation and meltdown behaviour and alkali circulation result in less efficient gas-solid contact, unstable burden descent and unstable hot metal quality requiring a higher blast furnace fuel rate that results in a lower productivity.
There are several mineralogical factors to be considered that impact on these behaviours. Improving any of the following behaviours improves the blast furnace process and can increase blast furnace productivity and efficiency.
First of all, acid materials—namely materials containing substantial amounts of silica or alumina, react strongly with alkalis to bind them in forms more stable than carbonates or cyanides. Alkalis circulating in the form of carbonates or cyanides deposit in the shaft to block gas flow, cause scaffolds to form on the walls, clustering of the ore layers, and react with coke or agglomerates causing degradation. Addition of silica, in the form of gravel, for example is effective in adjusting the final tapped slag composition, however the particle size of such gravel, generally charged at +6 mm, yields a rather low surface area for gas-solid reaction. Due to the low surface of bulk additives, the reaction with alkalis is not maximised.
Secondly, when the agglomerates begin to melt down, acidic slags are the first to flow from iron ore agglomerates. The slags require fluxing by network-breaking oxides such as CaO and MgO which may be added as bulk solids such as lumpy limestone, converter slag, dolomite or olivine, typically in particulate sizes much greater than 6 mm. However, due to the heterogeneous distribution of the fluxing particles extreme slag compositions may be present resulting in high viscosity slags blocking gas flow and potentially causing clustering of pellets, or in worst case, refreezing of slag causing extreme channelling of gas and hanging.
Thirdly, the clustering of iron ore agglomerates, due to either solid-state sintering of iron or low melting point surface slag can be alleviated by application of a high melting point mineral layer at the contact points between agglomerates. Clustering has been reduced in the DR process by applying high-melting point minerals to the DR pellet surface.
A final consideration that is not related to the chemical behaviour of the furnace is the water spraying typically used to minimise dusting in transport. Moisture in the pellets is to be avoided as it depresses blast furnace top gas temperatures which in some cases requires more fuel and therefore lowers blast furnace productivity. Dust suppression is also important in the blast furnace process because dusts escaping with blast furnace gas must be recovered and disposed of. Such dusts, commonly called flue dusts, are both a loss of iron units and expensive to dispose of or recycle. Furthermore, reducing the dusting in transport lessens iron unit losses and improves the environmental aspect of blast furnace ironmaking.
U.S. Pat. No. 4,350,523 discloses iron ore pellets when used in a blast furnace reduces the coke and fuel rates and also frequency of slips and the fluctuations in the blast furnace process. According to the document the reducibility of the pellets (the so called retardation of reduction) in the high temperature zone is improved by increasing the porosity and pore diameters of the individual pellets. The pellets are manufactured by adding a combustible material to the pellets during the pelletizing process before firing of the pellets.
RU 173 721 discloses the problems of loosening and breakage of pellets in the upper part of a reducing unit and the problems of sticking of pellets during the intensive formation of metallic iron in the middle and lower part of the furnace shaft. In accordance with the teachings of the document the problems are reduced by applying a coating of CaO and/or MgO-containing materials to the green pellets just prior to firing. By altering the basicity of the surface layer, the reduction properties of the pellets are improved.
Although blast furnace efficiency and productivity has steadily improved through various means, the process can still be improved. The object of the present invention is therefore to provide a method that improves fuel efficiency and stability, and thereby production rate, in such a way that does not alter the fired pellet reducibility or reduction degradation properties. The means to provide such improvements are to reduce the amount of gas channeling, slipping and dust formation via improved slag formation and melting behaviour, reduction of the degree of clustering of iron ore agglomerates, and reduction or modification of the circulation of alkalis in the blast furnace.
Accordingly, the development and proposals suggested herein surprisingly have shown to improve the efficiency and the production rate in blast furnaces.